(1) Field of the Invention
The present invention relates to a process for the preparation of an aqueous slurry of zeolite which is excellent in the suspension stability, especially in the combination of static stability and dynamic stability and which has a relatively low vicosity stably even at normal temperatures and shows a good flowability. Furthermore, the present invention relates to a process for the preparation of a zeolite builder slurry which can advantageously be used for the manufacture of detergents.
(2) Description of the Prior Art
It has been known from old that a water-insoluble alkali metal aluminosilicate such as zeolite has an excellent metal ion sequestering capacity and exerts a high buffering action and a good re-contamination preventing effect under alkaline conditions and that the water-insoluble alkali metal aluminosilicate can effectively be used as a detergent builder by dint of these excellent characteristics.
However, there are involved various problems to be solved in manufacture and transportation of zeolite builders. Zeolite is a substance having a dilatant characteristic and it is difficult to sufficiently remove water from crystallized zeolite by such means as filtration, and if the filter cake of zeolite is allowed to stand still under no application of an external force, the filter cake is converted to a mud-like product. Accordingly, in order to obtain a dry powder of zeolite, it is necessary to adopt costly drying means such as spray drying. Zeolite particles tend to agglomerate under drying, but from the viewpoint of the builder characteristics, it is preferred that zeolite particles be as small as possible. Accordingly, the spray-dry product of zeolite should be subjected to a pulverizing treatment which requires a long time. Moreover, the fine dry powder of zeolite is very bulky, and therefore, the transportation expenses are increased and the working environment is polluted by dusting on handling or transportation of the fine dry powder of zeolite.
As means for eliminating the foregoing disadvantages caused when powdery zeolite is handled, there has been adopted a method in which crystallized zeolite is handled in the form of an aqueous slurry. However, also in this case, there arise other problems to be solved. Zeolite particles have a tendency to precipitate in water and when an external force is applied by vibrations or the like during transportation, the particles aggregate densely to form a very hard precipitation cake. Therefore, when zeolite is handled in the form of an aqueous slurry, it often happens that even discharge of zeolite from a vessel becomes difficult.
Many trials have been made to form an aqueous liquid of zeolite. In these trials, various surfactants or dispersants are used so as to prevent precipitation of zeolite particles. For example, there are known a method in which a carboxyl group-containing water-soluble polymer such as carboxymethyl cellulose, a water-swellable laminar clay mineral such as bentonite or a non-ionic surface active agent such as an ethylene oxide adduct of a higher alcohol is added as a stabilizing agent to an aqueous slurry of zeolite (see U.S. Pat. No. 4,072,622), a method in which a water-soluble alkali metal salt such as sodium carbonate and a non-ionic surface active agent are added in combination to an aqueous slurry of zeolite for attaining a stabilizing effect (see Japanese Patent Application Laid-Open Specification No. 155200/79) and a method in which an organic flocculating agent such as polyacrylamide, polyacrylic acid or an acrylic acid copolymer is added to an aqueous slurry of zeolite (see Japanese Patent Application Laid-Open Specification No. 84533/80).
These known methods are still insufficient in that when a dispersion stabilizer such as mentioned above is added to an aqueous slurry of zeolite, the viscosity of the slurry is extraordinarily increased and in order to keep this aqueous slurry flowable, it is necessary to maintain the aqueous slurry at a high temperature or perpetually conducted gentle stirring. Moreover, these known methods are defective in that the effect of preventing precipitation of particles when the resulting aqueous slurry is allowed to stand under stationary conditions, that is, the static stability, and the effect of preventing precipitation of particles when the resulting aqueous slurry is allowed to stand under dynamic conditions, that is, under application of vibrations or other external forces caused on transportation, are not satisfactory.
Japanese Patent Application Laid-Open Specification No. 64504/79 discloses an aqueous slurry of zeolite stabilized against precipitation, which is formed by dispersing zeolite particles in an aqueous solution of sodium silicate. This aqueous slurry shows a good stability for short-time storage, but when it is stored for a long time, type A zeolite is changed to type Pc zeolite or analcime and the ion exchange capacity is drastically reduced, and occurrence of precipitation and aggregation becomes conspicuous.